In the past, lens modules for image reading systems mounted in comparatively thin devices, such as simple digital cameras, webcams for PCs, and other handheld devices, have been required to be short and inexpensive rather than capable of high optical performance. In order to satisfy these requirements, imaging lenses have conventionally been formed of a single lens component. In such devices, image pickup devices such as CCDs have been small for receiving a small image and they have been of relatively low resolution. Fewer lens components and lens elements in the imaging lenses of the lens modules has enabled the lens modules to be very small.
Recently, larger image pickup devices with higher resolution have been developed. Large aberrations caused by the use of a single lens component or single lens element lens module have prevented sufficiently improved optical performance from being achieved with these higher resolution and larger image pickup devices. Therefore, it has become necessary to develop an imaging lens that takes advantage of the higher resolution and larger size of recent image pickup devices. This resulted, for example, in a three-component imaging lens being proposed with all the lens components made of plastic and including at least one aspheric lens surface, as described in Japanese Laid-Open Patent Application H10-048516. However, the lens components disclosed in this published Japanese application are not all sufficiently small and inexpensive. Additionally, the optical performance of this imaging lens, particularly with regard to correcting for curvature of field and distortion, is not satisfactory.
Furthermore, for image reading using a PC or a handheld device, a small image pickup device has been used that requires the principal ray to strike the image detecting surface of the image pickup device at an angle of approximately fifteen degrees or less to the surface normal. However, due to recent improvements in image pickup devices, it has become acceptable for the principal ray to strike the image detecting surface at a larger angle of incidence, for example, approximately twenty-five degrees or greater. Therefore, an imaging lens where various aberrations can be excellently corrected even if the principal ray strikes the detecting surface at larger angles of incidence is desired.
The present invention is a single focus lens formed of three lens components that is advantageously used with recently improved image pickup devices as in, for example, small digital cameras, image reading devices connected to PCs, and other handheld devices. The single focus lens is compact and of inexpensive construction, and achieves a high optical performance, with particularly excellent correction of curvature of field and distortion.